A throttle controls the flow of air, or air and fuel, inducted into an internal combustion engine, and thereby controls the power produced by the engine. Engine power defines the speed of the engine or vehicle to which it is attached, under a given load condition, and thus, reliable control of the throttle setting is important.
In prior art mechanical systems, a direct mechanical linkage controlled the throttle, typically in the form of a cable running from the accelerator pedal, operable by the user of the vehicle, to the throttle valve. Although mechanical linkages are simple and intuitive, they are not readily adapted to electronic control of an engine such as may be desired in sophisticated emissions reduction systems or for features such as automatic vehicle speed control. For these purposes, the mechanical linkage may be replaced with electrical wiring carrying throttle signals from a position sensor associated with the accelerator pedal to a throttle controller operating a throttle actuator (typically an electric motor) for actuating the throttle valve.
While electronic control without mechanical linkages allows for the introduction of a variety of desirable control features, electronic control also makes the operation of the throttle dependent upon the throttle signals to the throttle controller, which controls the throttle actuator. These throttle signals may pick up errors due to noise or otherwise. Those errors can have undesirable effects on the control of the throttle, as discussed below.
As shown in FIG. 1 (Prior Art), a typical throttle includes a conduit, through which air (or an air-fuel mixture) flows, and a rotatable throttle plate that in part determines the flow rate based on its position within the conduit. In between a closed position, in which the throttle plate prevents nearly all flow through the conduit, and a wide-open position, in which the throttle plate allows a maximum flow rate, there is typically a default position for the throttle plate. The default position is a position of the throttle plate in which a relatively small flow rate is allowed (i.e., where the throttle plate is closer to closed than open).
Under normal operating conditions, the position of the throttle plate is positioned by the throttle actuator (i.e., electric motor). The throttle actuator is typically coupled to the throttle plate by a pair of gears in between which exists lash. (In other cases, the throttle actuator and throttle plate can be coupled by other linking elements that also have lash, such as a belt.) However, the throttle plate is also coupled to a spring mechanism which biases the throttle plate towards the default position. If for some reason the throttle actuator is unable to control the position of the throttle plate (i.e., the throttle actuator produces no output torque), the spring mechanism moves the throttle plate to the default position. Because there is a small amount of flow through the conduit in the default position, the vehicle remains (at least partly) operational when this occurs.
Although the spring mechanism is necessary for allowing partial operation of the vehicle when the throttle actuator is malfunctioning, the spring mechanism complicates the electronic control of the throttle. Proper control of the throttle under normal operating conditions (i.e., when the throttle actuator is properly operating) requires that the throttle actuator compensate for (i.e., counteract) the torque of the spring mechanism. Typically, this compensation is effected by the introduction, into the throttle signals, of a feedforward component.
Generation of the proper feedforward component when the throttle plate is near the default position is difficult, however, for two reasons. As shown in FIG. 2 (Prior Art), the torque provided by the spring mechanism changes in a discontinuous manner when the throttle plate crosses over the default position. Additionally, because the spring mechanism biases the throttle plate in opposite directions when the throttle plate is on opposite sides of the default position, the gears coupling the throttle plate and the throttle actuator experience a relative shift due to the gear lash as the throttle plate moves through the default position.
Because of the interaction of the spring mechanism, the gear lash and the feedforward component, exact control of the positioning of the throttle plate near the default position is difficult, and undesirable fluctuation of the throttle plate can occur near the default position. This particularly becomes a problem if noise (i.e., duty cycle variation) occurs within the throttle command signal when the throttle plate is at or very close to the default position, such that the throttle signals are effectively commanding the throttle plate to shift back and forth across the default position. Under these circumstances, the throttle plate can experience rapid, undesirable fluctuation that can result in annoying rattling of the throttle plate.